Rated capacities
We've already discussed the total output wattage rating of each of the power supply units in the spotlight today, but that's only one component of the output rating story. Modern PSUs divide power across three main lines at 3.3, 5, and 12 volts, with each of those lines carrying a maximum current rating. As if that weren't complicated enough, most PSUs also spread 12V power over multiple individual lines.

Most of these PSUs spread 12V power over three or four rails, usually with an identical maximum current rating for each. BFG Tech's ES-800 is an exception, thanks to a 22A maximum current rating for two of its 12V rails and a beefier 36A rating for the others. Meanwhile, both OCZ and PC Power & Cooling eschew the idea of multiple rails completely. Instead, the EliteXStream and Silencer consolidate 12V power under a massive single rail. Mushkin's XP-800AP can also be switched between single and quad-rail 12V modes, although we didn't observe much of a performance difference between the two.

Don't get too married to these current ratings, though; they're only the tip of the iceberg. To get a handle on each PSU's true capacity, we have to determine the maximum output wattage of each voltage line. The math is easy enough thanks to Ohm's Law, which allows us to calculate wattage given voltage and amperage. However, power supply units with multiple rails are typically limited by how much power can be spread across those multiple lines—a total that's usually less than the sum of each rail's output capacity. Most PSUs also place limitations on how much power can be shared between the 3.3V and 5V lines. And then there's the maximum output wattage across the 3.3, 5, and 12V lines, which often falls short of the PSU's total output rating in order to reserve capacity for lesser-used voltage lines like the -12V and 5V standby rails.

To make sense of it all, we've put together a table showing the maximum output power for each PSU's 3.3, 5, and 12V rails. Where applicable, we've also indicated the maximum combined 3.3 and 5V power, the maximum combined 12V power, and how much wattage the PSU can spread across all three main rails.

Sick of tables yet? I thought so. But bear with me, because maximum output power ratings tell us more about a power supply's capabilities than anything else.

At the high end of the wattage spectrum, the units from BFG, Mushkin, and OCZ can deliver a full 800W across their 3.3, 5, and 12V rails. BFG's ES-800 has the highest combined 12V output rating at 780W, followed closely by the Mushkin XP-800AP at 768W. However, it's the EliteXStream that can support the heaviest combined 3.3 and 5V loads at up to 180W.

In the middle of the range we've assembled, the 750W models from OCZ and Zalman offer the same 720W maximum 12V output capacity. The ZM-750HP's combined 3.3, 5, and 12V output is limited to only 728W, though, and its 160W combined 3.3 and 5V maximum falls 10W short the Silencer.

Turning our attention to entrants in the 600W range, the Thermaltake looks to be the most ambitious. The Purepower boasts a 576W maximum output on its 12V line and a whopping 180W of combined 3.3 and 5V capacity, despite having the lowest total output capacity of the bunch. Enermax has biased its PRO82+ towards 12V output capacity, where it can feed up to 600W, while ePower provides a little extra combined 3.3 and 5V power.